1. Field of the Invention
The present invention relates to an electromagnetic wave absorber and a process of producing same, and more particularly, to an electromagnetic wave absorber which is so designed as to exhibit a stable electromagnetic wave absorption characteristic in a selected high-frequency band of 5 to 6 GHz and a process of producing such an electromagnetic wave absorber.
2. Description of the Related Art
Sheetlike material obtained by kneading a matrix, which may be rubber or various resins, for example, with soft magnetic powder and rolling the kneaded mixture into a sheet has been put to practical use as an electromagnetic wave absorber. Such an electromagnetic wave absorber is incorporated into various electric/electronic devices so as to absorb noise attributable to electromagnetic waves generated by these devices or propagated from other devices and thereby prevent malfunction etc. of the devices.
Recently, there is a tendency to use higher-frequency signals for driving electric/electronic devices and systems constituted by such devices. For example, use of the 5 GHz band in indoor wireless LAN is under study, and in the ETC system and DSRC (Dedicated Short Range Communications), the 5.8 GHz band is actually used.
Under the circumstances, the electromagnetic wave absorber incorporated into such systems is also required to have a good electromagnetic wave absorption characteristic in the high-frequency band.
To meet the requirements, Unexamined Japanese Patent Publication No. 2003-60383, for example, discloses an electromagnetic wave absorbing sheet as an electromagnetic wave absorber for use in high-frequency band, in which a matrix of polypropylene is filled with 25 to 40 volume % of soft magnetic powder having an average particle diameter of, for example, 3 μm or less.
Meanwhile, a very wide frequency range of electromagnetic waves is currently used in various applications. Accordingly, the electromagnetic wave absorbers need to exhibit excellent absorption characteristics in the frequency bands of their respective applications. Namely, it is not necessary that one electromagnetic wave absorber should have an excellent absorption characteristic over such a very wide frequency range. Instead, the electromagnetic wave absorber may have an excellent absorption characteristic in a particular selected frequency band, and then the absorber is sufficiently useful for systems using that frequency band.